Bridge for semiconductor internal node

ABSTRACT

A method and apparatus for forming connections within a semiconductor device is disclosed. The semiconductor device incorporates a contact bridge between transistor contacts in close proximity. The contact bridge comprises a plurality of metal pillars each having a lower end in electrical contact with first and second transistor elements, respectively; one or more intermediate metal pillars disposed between and in electrical contact with an upper end of the metal pillars; and one or more separation regions of dielectric disposed below the intermediate metal pillar and between the lower ends of the first and second metal pillars.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of commonly-owned, copending U.S. patentapplication Ser. No. 11/468,102 filed on entitled BRIDGE FORSEMICONDUCTOR INTERNAL NODE, and filed on Aug. 29, 2006.

FIELD OF THE INVENTION

The present invention generally relates to the field of semiconductorfabrication. More particularly, the present invention relates to amethod and apparatus for forming connections within a semiconductordevice.

BACKGROUND

In semiconductor design, particularly SRAM design, it is often desirableto create a contact bridge between contacts in very close proximity.FIG. 1 shows a plan view of a schematic representation of design patternfor an exemplary semiconductor device 100 typically disposed on anintegrated circuit (IC). The device 100 is formed on a silicon substrate(not shown). Using standard lithographic methods that are well known inthe art, an etch is performed on the silicon substrate, resulting inreduced thickness of the silicon substrate (known as a shallow trench),except for the places where the lithographic method prevented the etchfrom occurring. These areas 104 which were not etched away are referredto as silicon traces. Continuing the process, a dielectric layer 114 isapplied to cover the then exposed upper surface of the device 100. Then,the dielectric layer 114 is partially removed, typically by polishing,so that only the upper surface of each silicon trace 104 is exposed.Continuing, a layer of polycrystalline silicon is applied to cover thethen exposed upper surface of the device 100. Using standardlithographic methods that are well known in the art, an etch isperformed on the surface to form a plurality of polycrystalline silicon(referred to as polysilicon) lines or traces 106. FIG. 1 shows therelationship between the silicon traces 104 and polycrystalline silicontraces 106. Continuing, a dielectric layer 112 is applied across thethen exposed upper surface of the device 100. Portions of the dielectriclayer 112 are then removed using standard lithographic methods leavingsidewall spacers 112A and 112B on either side of silicon traces 106.Next a dielectric layer 116 is applied across the then exposed uppersurface of the device 100. Finally, according to the prior art,conductive contacts 108, 109, referred to as a CA (contact area), andcontact area rectangle structures 110, referred to as a CAREC herein,are put in place, as described below. The conductive contacts 108, 109make electrical contact with the silicon traces 104 and polysilicontraces 106, respectively. It is sometimes desirable to connect a gate ofone transistor to a source or drain of another transistor in closeproximity. In order to make this connection, the CAREC 110 can be used.The CAREC 110 is a form of well known local interconnect wiring. To formthe contacts 108, 109 and CAREC 110, the dielectric layer 116 is etchedaway to form cavities, such as cavity 115 in FIG. 2. Then a conductivematerial such as tungsten is deposited in the cavities to formconductive pillars. These pillars form the CAs 108,109 and CARECs 110.

The semiconductor device 110 is generally comprised of an arrangement ofmany transistors on a silicon substrate. The plurality of transistors isformed by the arrangement of the silicon traces 104 and the polysilicontraces 106, which form the source or drain of each transistor. As shownin FIG. 1, the contacts 108 are in electrical contact with silicontraces 104 and contacts 109 are in electrical contact on the polysilicontraces 106.

It should clearly be understood that FIG. 1 illustrates but an extremelysmall (microscopic) portion of an integrated circuit (IC) device, letalone a semiconductor wafer comprising a large plurality of suchdevices. For example, what is shown may have a width of only a fewmicrons (pm) of a semiconductor wafer having a diameter of severalinches. Also, in “real life” things are not so neat and clean,rectilinear and uniform as shown. However, for one of ordinary skill inthe art to which the invention most nearly pertains, this and otherfigures presented in this patent application will be very useful, whentaken in context of the associated descriptive text, for understandingthe invention.

The semiconductor device 100 shown in FIG. 1 (as well as in the otherFigures) is fabricated utilizing conventional processing steps wellknown to those skilled in the art. Since such techniques are well knownand are not critical for understanding the present invention, a detaileddiscussion of the same is not given herein. It will be understood thatvarious steps and materials have been omitted, for illustrative clarity,such as seed layers, adhesion layers, cleaning steps and the like.

FIG. 2 shows a cross sectional view of a portion of semiconductor device100, as viewed along line A-A of FIG. 1, showing the details of CAREC110. The CAs 108,109 (shown in FIG. 1) and the CARECs 110 are formed byusing a selective etch to etch cavities in the dielectric 116 until thedesired silicon or polysilicon layer is reached. Then a conductivematerial such as tungsten is deposited in the cavities to formconductive pillars. These pillars form the CAs 108,109 and CARECs 110.

Referring again to FIG. 2, CAREC 110 can be formed by first performing areactive ion etch on the desired area to remove a portion of dielectriclayer 116. This etching forms a cavity 115 that is filled with aconductive metal, such as tungsten. The result is shown in FIG. 2, inwhich CAREC 110 is formed over polysilicon trace 106. Polysilicon trace106 serves as the gate of a transistor. Adjacent and on either side ofpolysilicon trace 106 are sidewall spacers 112A and 112B. The sidewallspacers 112A and 112B are important during the etching process toprevent damage to the doping implants under silicon trace 104 andpolysilicon trace 106. Ideally, sidewall spacers 112A and 112B should beapproximately symmetrical. However, because, the etching of the cavitiesforming pillars CAs 108,109 and CARECs 110 occurs at the same time,sidewall spacer 112A gets more eroded by the etch process, since on theleft side, the cavity 115 is deeper so as to reach trace 104 as comparedto the right side where the cavity 115 goes down to the trace 106. Theresult is damage to spacer 112A, and a portion of the upper surface ofsilicon trace 104. This damage may adversely remove dopants that wereput there prior to the etching step, during the implant phase of themanufacturing process. This creates a high resistance element, whichdegrades the performance of the semiconductor.

There are multiple drawbacks to this process. First, the etching processworks on a global level. Therefore, it is desirable to have oneconsistent shape for etching, so that dielectric material will be etchedat a similar rate. The CARECs 110 have approximately double the area ofthe CAs 108 and 109. The area being etched effects the rate of etch.Therefore, with shapes of various areas being etched, the etchingprocess is not as consistent as it would be if one uniform shape wasused. Second, etching the CAREC may compromise the sidewall spacer ofthe transistor and remove dopants, resulting in degraded semiconductorperformance. As the demands of technology require more complexfunctionality in products having size constraints, such as portableelectronics products, there is an increasing need to fit moretransistors on a semiconductor device. Therefore, what is needed is animproved connection method that allows the flexibility of connectingcontacts in close proximity, provides a consistent etch shape, and doesnot compromise the integrity of a sidewall spacer or cause unwantedremoval of dopants.

SUMMARY OF THE INVENTION

According to the present invention, there is disclosed a method forfabricating a connection between two transistor elements on asemiconductor substrate. The method comprises the following steps:providing the semiconductor substrate with a silicon line forming afirst transistor element, a polysilicon line forming a second transistorelement, a first side spacer on one side of second transistor elementand a second side spacer on an opposite side of the second transistorelement, and a dielectric layer overlying the first transistor element,the second transistor element, and the first and second side spacers;applying a layer of photo resist over an upper surface of the dielectriclayer; photo patterning said photo resist layer to form at least firstand second contact areas with an area of photo resist therebetween;forming at least first and second cavities corresponding to the at leastfirst and second contact areas extending through the photo resist layerto the dielectric layer with a region of the photo resist remainingtherebetween; etching the dielectric layer through the at least firstand second cavities to form at least first and second contact cavitiesin the dielectric layer and concurrently reducing the thickness of thephoto resist layer and resist region to form a first intermediate cavitybetween first and second contact cavities and a first separation regionof the dielectric layer between the first and second contact cavities;further etching the dielectric layer until the first contact cavitycontacts the first transistor element, the second contact cavitycontacts the second transistor element, the first intermediate cavityextends between contact cavities and down to the first separation regionof the dielectric layer between contact cavities; and depositingconductive metal in the first and second contact cavities and in theintermediate cavity to form a first, a second and an intermediateconductive metal pillar.

Further according to the present invention, the method includes joiningthe first, second and intermediate conductive metal pillars together atan upper end top thereof and placing them in electrical contact with thefirst transistor element and the second transistor element at a bottomend thereof and isolating the intermediate conductive metal pillar fromthe first side spacer with the first separation region to form a doubleCA bridge structure.

Still further according to the present invention, the method includesselecting the conductive metal from the group consisting of tungsten andcopper.

Also according to the present invention, the method includes etchingwith a reactive ion etch process.

Yet further according to the present invention, the step of depositingconductive metal in the first and second contact cavities and theintermediate cavity creates an excess layer of conductive metal acrossthe upper surface of the dielectric layer. The excess conductive metalcan be removed from the upper surface of the dielectric layer via achemical mechanical polish.

Further according to the present invention, the method includes forminga triple CA bridge structure with first, second and third contactcavities, first and second intermediate cavities and first and secondseparation regions for isolating first, second and third side spacers.Then depositing conductive metal in the first, second and third contactcavities and in the first and second intermediate cavities to formfirst, second and third conductive metal pillars and a first and secondintermediate conductive metal pillars. This causes the firstintermediate conductive metal pillar to be disposed between the firstand second conductive metal pillars and the second intermediateconductive metal pillar to be disposed between the second and thirdconductive metal pillars. Also the first separation region is disposedbetween the first and second conductive metal pillars and the secondseparation region is disposed between the second and third conductivemetal pillars, thereby forming a triple CA bridge structure.

Yet further according to the present invention, a quad CA bridgestructure can be formed by the steps of forming first, second and thirdand fourth contact cavities, first, second and third intermediatecavities and first, second and third separation regions for isolatingthe first, second, third and fourth side spacers. Then depositingconductive metal in the first, second, third and fourth contact cavitiesand in the first, second and third intermediate cavities to form first,second, third and fourth conductive metal pillars and first, second andthird intermediate conductive metal pillars. The first intermediateconductive metal pillar is disposed between the first and secondconductive metal pillars, the second intermediate conductive metalpillar is disposed between the second and third conductive metalpillars, the third intermediate conductive metal pillar is disposedbetween the third and fourth conductive metal pillars. The firstseparation region is disposed between the first and second conductivemetal pillars, the second separation region is disposed between thesecond and third conductive metal pillars, and the third separationregion is disposed between the third and fourth conductive metal pillarsthereby forming a quad CA bridge structure. When the first, second,third, and fourth conductive metal pillars are arranged linearly, alinear quad CA bridge structure is formed. Also when the firstconductive metal pillar is arranged at a right angle in relation to thesecond, third, and fourth conductive metal pillars, a quad CA ‘L’ bridgestructure is created.

Also further according to the present invention, there is disclosed asemiconductor device having a contact bridge between transistor contactsin close proximity. The contact bridge comprises at least first andsecond metal pillars each having a lower end in electrical contact withfirst and second transistor elements, respectively; at least a firstintermediate metal pillar being disposed between and in electricalcontact with an upper end of the first and second metal pillars; and atleast a first separation region of dielectric disposed below firstintermediate metal pillar and between the lower ends of the first andsecond metal pillars.

Still further according to the present invention, the semiconductordevice incorporates the first and second metal pillars, the intermediatemetal pillar and the first separation region being arrangedsubstantially vertically; the first metal pillar is oriented above andin contact with the first transistor element; the second metal pillar isoriented above and in contact with the second transistor element; firstand second sidewall spacers are disposed on opposite sides of the secondtransistor; and first separation region of dielectric isolates the firstsidewall spacer from the first intermediate metal pillar.

Also further according to the present invention, the contact bridgefurther comprises:

at least first, second and third metal pillars each having a lower endin electrical contact with first, second and third transistor elements,respectively; at least first and second intermediate metal pillars beingdisposed between and in electrical contact with an upper end of thefirst, second and third metal pillars; and at least first and secondseparation regions of dielectric disposed below the first and secondintermediate metal pillars and between the lower ends of the first,second and third metal pillars.

Still further, the semiconductor device has: the first, second and thirdmetal pillars, the first and second intermediate metal pillars and thefirst and second separation regions are arranged substantiallyvertically; the first metal pillar is oriented above and in contact withthe first transistor element; the second metal pillar is oriented aboveand in contact with the second transistor element; the third metalpillar is oriented above and in contact with the third transistorelement; first and second sidewall spacers are disposed on oppositesides of the second transistor; and the second sidewall spacer and athird sidewall spacer are disposed on opposite sides of the thirdtransistor; and the first separation region of dielectric isolates thefirst sidewall spacer from the first intermediate metal pillar and thesecond separation region of dielectric isolates the third sidewallspacer from the third intermediate metal pillar.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a schematic of a design pattern of a prior artsemiconductor device.

FIG. 2 is a cross section view of a portion of the semiconductor deviceof FIG. 1 showing the CAREC and the damage to a spacer and a portion ofthe upper surface of a silicon trace.

FIG. 3 is a plan view of a design pattern placed on the upper surface ofa semiconductor device prior to etching the cavities forming thecontacts and twin CA structure, according to the present invention.

FIGS. 4-12 is a view through line A-A of FIG. 3 showing the varioussteps required to form the twin CA bridge structure.

FIGS. 13A-13C show alternate embodiments of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 3 shows a schematic illustration of an intermediate stage in theconstruction of an exemplary embodiment semiconductor device 300 of thepresent invention. Semiconductor device 300 is similar to semiconductordevice 100 of FIG. 1, with the exception that twin CA bridge structures350 described below have been used to replace the CARECs 110 of FIG. 1.That is to say, polysilicon lines 306 are similar to polysilicon lines106 of FIG. 1, silicon lines 304 are similar to silicon line 104 of FIG.1, and contacts 308, 309 are generally similar to contacts 108, 109 ofFIG. 1.

To form twin CA bridge structures 350, two particular contact areas ofinterest, indicated as 308A and 309A are placed in close proximity toeach other. As illustrated in FIG. 3, contact area 308A is placed on asilicon trace 304, and contact area 309A is placed on a polysilicontrace 306.

Where previously a CAREC 110 (see FIG. 1) was used to join two traces304 and 306 in close proximity, the twin CA bridge structure 350 asdescribed below, interconnects the two traces 304, 306, without theproblems of the CAREC 110. In the semiconductor device 300 of FIG. 3,all of the contact areas, i.e. CAs 308 and 309, are preferably formedwith substantially the same cross sectional area and with the samesquare shape. Using the same configuration, makes the etching process ofthe CAs more efficient and predictable because, being of substantiallythe same size and geometry, they etch at the same rate and to the samedepth in essentially the same amount of time.

FIG. 4 shows an intermediate stage of the construction of asemiconductor device 300 where the first, second and third dielectriclayers 314, 312 and 316 have already been applied. A layer 318 of photoresist is applied having a thickness in the range of 200 nanometers to600 nanometers.

Next, as shown in FIG. 5, a view through line B-B of FIG. 3, usinggraphic methods, first and second, substantially identical CA shapedcavities 320A and 320B, are formed through the surface of resist 318.One of the CAs 320A is aligned with polysilicon line 306. The other oneof the CAs 320B is aligned silicon line 304 and to the left of line 306.Between CA 320A and CAs 320B, a sliver region 318A of resist alignedabove a first side spacer 312A of a dielectric 312 remains. This sliverresist region 318A is important for forming the twin CA bridge structure350 of the present invention, as will be described hereinafter.

FIG. 6 shows the beginning of an etch step where cavities 321A and 321Bare being etched down into dielectric layer 316. The resist layer 318Bis also getting worn away to form a cavity 320C between cavities 320Aand 320B by the etch process. The cavity 321C actually creates a singlecavity with cavities 321A and 321B. Note that the sliver resist region318B is etched away at a faster rate than the rest of the resist, due tothe increased ratio of surface area to volume of that feature, ascompared with the rest of the photo resist 318.

FIG. 7 shows the etch process further along from FIG. 6. Now, cavities321A and 321B project further into dielectric layer 316 and cavity 321Cis beginning to get longer in the resist layer 318. At this stage of theprocess, the resist layer 318 is very thin but still intact. However,the sliver resist region 318B is now very thin.

FIG. 8 shows the sliver resist region 318A completely removed fromcontinued etching. The rest of the resist 318, while thinner, is stillintact. Cavities 321A and 321B are still deeper and intermediate cavity321C extends downward between cavities 321A and 320B. Directly belowintermediate cavity 321C is a separation region 316A of dielectric layer316 between cavities 321A and 320B.

FIG. 9 shows a portion of dielectric layer 316 in between cavities 321Aand 321B removed by further etching. Cavities 321A and 321B are joinedat the upper portion, as the dielectric layer 316 is being etched away,by intermediate cavity 321C. Because cavities 321A and 321B form contactareas when the fabrication process is complete, cavities 321A and 321Bare referred to as contact cavities. An important aspect of theintermediate cavity 321C is that it does not extend to the bottom ofcavities 321A and 321B.

FIG. 10 shows the step where the cavities 321A and 321B have beenfurther etched through the dielectric layer 316. Any remaining resist318 is removed, typically, by burning it off in an oxygen plasma. Cavity321B extends to the silicon layer 304. Cavity 321A extends to thepolysilicon trace 306. Cavity 321C, while being deeper, is in contactwith the sidewalls of cavities 321A and 321B and is separated by aseparation region 316A of dielectric 316 above the sidewall spacer 312A.

It can now be appreciated that the resist sliver 318B (See FIG. 5)provided initial protection of the dielectric layer 316 from the etchantin the area 321C between cavity 321A and 321B, and therefore cavity 321Cis not as deep as 321A and 321B. This construction step protected thetop of polysilicon trace 306 and spacer 312A and the silicon trace 304in the proximity of spacer 312A from undesired etching. By protectingthe spacer 312A, the polysilicon trace 306 and silicon trace 304, thedopants applied to the silicon trace 304 during the implant phases (notdescribed) are preserved. Moreover, the sidewall spacer 312A iscompletely intact and able to control further implant of dopants. Bycomparison, in the prior art, see FIG. 2, a portion of the polysilicontrace 306, sidewall spacer 312A, and a portion of silicon trace 304 areroutinely destroyed during the etching process.

FIG. 11 shows where a conductive material 322 such as tungsten or copperhas been deposited over the semiconductor. The tungsten fills cavities321A, 321B and 321C and forms a twin CA bridge structure 350 comprisingtungsten pillars 324A, 324B and 324C that are joined at the top thereof.The deposition of tungsten also forms a layer 322 on top surface 316B ofthe dielectric layer 316. This metal layer 322 shorts all the contacts,and must be removed for a properly functioning semiconductor device.

FIG. 12 shows the excess tungsten layer 322 removed by conventionalmeans such as with a chemical mechanical polish (CMP). The end result isthe twin CA bridge structure 350 which provides electrical contactbetween silicon trace 304 and polysilicon trace 306. The intermediatepillar 324C, see FIG. 11, is filled with tungsten and forms a bridgeconnecting pillars 324A and 324B.

FIGS. 13A-13C show alternate embodiments of the present invention thatextends this concept to more than two contacts bridged together. Thejoined CA bridge structure of the present invention can have more thantwo contacts.

FIG. 13A shows a cross sectional view of a joined CA structure 360 withthree pillars of conducting material 328A, 328B, and 328C joinedtogether, using the same technique that was described in detail for thetwin CA structure 350. Pillar 328A is in contact with trace 314, pillar328B is in contact with silicon trace 304 and pillar 328C is in contactwith polysilicon silicon trace 306. First and second intermediateconductive metal pillars 328 D and 328E are formed at the upper end ofthe three pillars of conducting material 328A, 328B, and 328C. The firstintermediate conductive metal pillar 328D is disposed between the firstand second conductive metal pillars 328A and 328C and the secondintermediate conductive metal pillar 328E is disposed between the secondand third conductive metal pillars 328D and 328B. A first separationregion 330A of dielectric 316 is disposed between the first and secondconductive metal pillars 328A and 328C and a second separation region330B of dielectric 316 is disposed between the second and thirdconductive metal pillars 328C and 328B, thereby forming a triple CAbridge structure.

FIG. 13B shows a plan view of a schematic design pattern for a structureemploying four contacts in a right angle pattern. This joined CAstructure is referred to as a Quad CA structure, and is indicated as370. In particular, because the arrangement of contacts 308E, 308F, 308Gand 308H forms a right angle, this joined CA structure 370 is referredto as a Quad CA ‘L’ structure. Using the techniques of presentinvention, as described in detail hereinbefore, contacts 308E-308H areinterconnected. Contact 308G may be solely an intermediate contact,serving only to join contact 308F to 308H, i.e., contact 308G may notdirectly contact a transistor element. The flexibility of this techniqueprovides allows for many possible interconnections, even if all thecontacts are not collinear, as is the case with this example. However,it is also possible to have a linear Quad CA structure, where allcontacts of the Quad CA structure are collinear. This is shown in FIG.13C. In this case, contacts 308J-308M are interconnected using a linearQuad CA structure 380. Regardless of the number of contacts used to makea joined CA structure, the contact cavities are arranged in a sequence.The sequence may be linear, or may be formed with at least one rightangle, or at angles other than 90 degrees. An intermediate cavity may beused to join two neighboring contact cavities. For example, in FIG. 13C,contact 308L has two neighboring contacts, 308K and 308M. There is anintermediate cavity in between the neighboring contact cavities. Theintermediate cavity provides electrical contact between the twoneighboring contacts once the metal deposition process has completed.

As is apparent from the aforementioned drawings and associated writtendescription, the present invention provides an improved method andapparatus for forming connections within a semiconductor device.

It will be understood that the present invention may have various otherembodiments. For example, while tungsten was used as the connectingmaterial in the examples provided, it is possible to use the method ofthe present invention with other materials, such as copper.

It is also understood, of course, that while the form of the inventionherein shown and described constitutes a preferred embodiment of theinvention, it is not intended to illustrate all possible forms thereof.It will also be understood that the words used are words of descriptionrather than limitation, and that various changes may be made withoutdeparting from the spirit and scope of the invention disclosed. Thus,the scope of the invention should be determined by the appended claimsand their legal equivalents, rather than solely by the examples given.

1. A semiconductor structure comprising: a transistor comprising a firsttransistor element and a second transistor element; a contact bridgeproviding electrical contact between the first transistor element andthe second transistor element, the contact bridge comprising: at leastfirst and second metal pillars each having a lower end in electricalcontact with the first and second transistor elements, respectively; atleast a first intermediate metal pillar being disposed between, andbounded by, the first metal pillar and the second metal pillar; and afirst separation region consisting of a single layer of dielectricdisposed below the first intermediate metal pillar and between the lowerends of the first and second metal pillars; and wherein: the first metalpillar is oriented above and in contact with the first transistorelement; the second metal pillar is oriented above and in contact withthe second transistor element; first and second sidewall spacers aredisposed on opposite sides of the second transistor element; and thefirst separation region of a single layer of dielectric isolates thefirst sidewall spacer from the first intermediate metal pillar, whereinthe region bounded by the first sidewall spacer, the first metal pillar,the intermediate metal pillar, and the second metal pillar consists onlyof the first separation region of a single layer of dielectric.
 2. Thesemiconductor device of claim 1, wherein the contact bridge furthercomprises: a third metal pillar, a third transistor element, and asecond intermediate pillar, wherein the first, second and third metalpillars each having a lower end in electrical contact with the first,second and third transistor elements, respectively; the first and secondintermediate metal pillars being disposed between and in electricalcontact with an upper end of the first, second and third metal pillars;and a second separation regions of a single layer of dielectric disposedbelow the second intermediate metal pillars and between the lower endsof the second and third metal pillars.
 3. The semiconductor device ofclaim 2 wherein: the first, second and third metal pillars, the firstand second intermediate metal pillars and the first and secondseparation regions are arranged substantially vertically; the firstmetal pillar is oriented above and in contact with the first transistorelement; the second metal pillar is oriented above and in contact withthe second transistor element; the third metal pillar is oriented aboveand in contact with the third transistor element; first and secondsidewall spacers are disposed on opposite sides of the second transistorelement; and the second sidewall spacer and a third sidewall spacer aredisposed on opposite sides of the third transistor element; and thefirst separation region of dielectric isolates the first sidewall spacerfrom the first intermediate metal pillar and the second separationregion of dielectric isolates the third sidewall spacer from the thirdintermediate metal pillar.
 4. The semiconductor device of claim 1,wherein said first and second metal pillars and said intermediate metalpillar are formed of a material selected from the group consistingessentially of tungsten and copper.